Dissecting Drowned Drumlin Fields

Earth and environmental sciences associate professor Peter Rosen said Boston Harbor is home to the nation’s only “drowned drumlin field,” a group of elongated hills formed under glacial ice.

He addressed roughly 100 students, faculty, staff and community members at the Marine Science Center (MSC) in Nahant on Wednesday as part of a monthly lecture series on topics ranging from the evolution of Boston Harbor to the seaweed habitats of marine animals.

Rosen — whose research examines eons of geological history, which can yield insights about the earth’s future — said about 200 drumlins pepper Metro West Massachusetts, “but what we see on land,” he noted “is a small fraction of the drumlin field that once existed.”

Typically, drumlins are up to two kilometers in length, 300 to 600 meters wide and less than 50 meters in height, with a characteristic shape — one steep side with a more gradual side opposite. Most drumlins, said Rosen, are now submerged offshore, drowned by rising sea levels caused by global warming.

Fourteen thousand years ago the “Laurentide Ice Sheet” (the glacier that covered this region) began melting in the Boston area, Rosen said. This caused massive amounts of sediment-laden water to flow into the harbor basin and raise the sea level about 100 feet higher than it is today, The sediment, called “Boston Blue Clay,” settled like a blanket over most of the glacial deposits.

The rise in sea level, Rosen said, was followed by a rapid decrease once the glacier melted: the land beneath it began to rebound upward, no longer compressed by the weight of the ice.

Some 12,000 years ago, sea level had reached its lowest point; continual melting of the glacier (now the Polar ice) gradually filled Boston Harbor with water, isolating many drumlins as islands.

Rosen said this more recent sea-level rise, at about 3mm a year, actually represents the slowest ascent in history. A comprehensive satellite study issued yesterday reports that the global sea-level increase was 12mm in the years between 2003 and 2010.

“How do we know about the early higher levels of the ocean?” Rosen asked. Answering his own question, he said: “From the Boston Blue Clay.”

Studies, he said, have turned up marine microfossils in the clay as far inland as Watertown, which is about 8 miles from the shoreline. These findings, Rosen said, indicate the presence of salt water — and thus a higher sea level — about 14, 000 years ago.

Rosen said the gradual rise in sea level can also be determined with the help of Boston Blue Clay. His team, for example, takes cores of the earth beneath salt marshes, which tend to grow at sea level. A chemical analysis called carbon dating can indicate how long ago that clay was at the surface.

“The impact of rising sea levels includes loss of land due to shore erosion, as well as an increase in the frequency and magnitude of coastal flooding,” Rosen explained. “This is especially important in low-lying areas such as Boston, where the land is mostly artificial fill just a few feet above sea level.”